1. Field of the Invention
The present invention relates to an optical information recording apparatus which records information in a recording medium in which information is recorded by utilizing holography, and an optical information reproducing apparatus which reproduces information in a recording medium in which information is recorded.
2. Description of the Related Art
Conventionally, holographic recording which records information in a recording medium by utilizing holography has been generally performed by overlapping inside the recording medium information light carrying image information and reference light for recording which construct light for recording, and writing a resultant interference pattern in the recording medium. When the recorded information is reproduced, the recording medium is illuminated with reference light for reproduction to cause diffraction attributable to the interference pattern which reproduces the image information (refer to Japanese Unexamined Patent Application Publication No. H11-311938).
Recently, volume holography, and more particularly, digital volume holography has been developed and is attracting attention in practical fields for super high density optical recording. The volume holography is a method of writing an interference pattern on the three dimensional basis by actively utilizing a recording medium even in its thickness direction, which is characterized in that diffraction efficiency is improved by increasing a thickness and in that an increased recording capacity can be achieved by utilizing multiplex recording. The digital volume holography is a computer oriented holographic recording method, in which image information to be recorded is limited to binary digital patterns while the recording medium and the recording method similar to of the volume holography are used. In this digital volume holography, for example, analog image information such as a picture is once digitized and developed into two dimensional digital pattern information, which is recorded as image information. When the information is reproduced, this digital pattern information is recorded and decoded to return to the original image information, which is to be displayed. Therefore, even when an SN ratio (signal-to-noise ratio) is somewhat poor when reproducing information, it is possible to recreate the original information very truly by performing differential detection and error correction on encoded binary data.
As a method of the holographic recording, a dominant method is a method to use an optical pickup apparatus comprising an optical system for performing information recording in the recording medium and information reproduction from the recording medium by adopting a disc-shaped recording medium similar to a CD (Compact Disc), a DVD (Digital Versatile Disc) and the like.
In general, in an optical disc apparatus, focus servo and tracking servo are performed by driving an objective lens in the optical pickup apparatus while rotating the disc-shaped recording medium.
In the optical disc apparatus, laser light is used as light for recording. When Q switch laser light is used as this laser light, information can be recorded in the recording medium in a short period of time even when the recording medium is rotating, since this Q switch laser light is laser light having large energy. Meanwhile, when semiconductor laser light is used as laser light, it is necessary to record information in the recording medium for a longer stretch of time than when the Q switch laser light is used, since this semiconductor laser light is laser light having small energy.
Therefore, when information is recorded in the recording medium by using the semiconductor laser light, it is necessary to change positions of illumination with the semiconductor laser light according to rotation of the recording medium.
As an example in which positions of illumination with the semiconductor laser light are changed as above, there is a case wherein an reflecting mirror arranged in an optical system to make laser light enter a recording medium is moved according to rotation of the recording medium (refer to International Publication No. 99/44195).
In the case that the reflecting mirror is moved according to rotation of the recording medium, which is described in the foregoing International Publication No. 99/44195, however, a drive means has to be connected to the reflecting mirror placed in the vicinity of the recording medium. Therefore, there is a risk that a weight of the optical system is increased, the reflecting mirror cannot be moved successfully, and therefore precision of information to be recorded in the recording medium becomes worse.
Such a phenomenon that the reflecting mirror cannot be moved according to rotation of the recording medium successfully might arise when information is reproduced from the recording medium.